What Is Fertility? – Sulfur

Sixth in a Series

The “secondary” plant nutrients—calcium (Ca), magnesium (Mg), and sulfur (S)—play a critical role in soil fertility and crop productivity. They are referred to as secondary nutrients because plants require them in moderate amounts compared to the primary macronutrients: nitrogen (N), phosphorus (P), and potassium (K).

To add to the confusion, some sources classify all six as macronutrients, further dividing them into primary macronutrients (N, P, K) and secondary macronutrients (Ca, Mg, S). Regardless of terminology, calcium, magnesium, and sulfur are essential nutrients that clearly influence soil health and crop performance and should be thoughtfully managed on agricultural properties whenever practical.

The Role of Sulfur in Crop Growth

Adequate sulfur availability contributes to strong plant vigor and a deep green color. When sulfur is deficient, crops may exhibit yellowing of young leaves, stunted growth, and delayed maturity, all of which can ultimately result in reduced yields.

Sulfur received relatively little attention until the past 20–30 years. Historically, one unintended benefit of less-regulated industrial emissions was that “acid rain” deposited sulfur—primarily as sulfuric acid—into agricultural soils. Animal manure applications have also long served as a meaningful sulfur source.

In addition, sulfur is naturally mineralized from soil organic matter. However, today’s high-yielding crop systems often remove sulfur faster than natural mineralization can replace it, making sulfur deficiency more common than in the past.

Sulfur and Nitrogen: A Critical Relationship

Sulfur is closely linked with nitrogen use in many crops. For example, corn typically uses sulfur and nitrogen at an approximate 1:10 ratio, meaning about 1 pound of sulfur is required for every 10 pounds of nitrogen taken up by the plant.

Sulfur can be applied alongside certain nitrogen fertilizers. While not as effective as commercial nitrification inhibitors, sulfur—when applied in the correct form—can function as both a nitrification inhibitor and a soil urease inhibitor. Applying sulfur with 28% or 32% liquid nitrogen at roughly a 10:1 ratio can help reduce nitrogen losses, keeping more nitrogen available in the soil for crop uptake. At the same time, it increases the likelihood that sufficient sulfur is present when the crop needs it most.

Sulfur and Soil pH Management

Sulfur can also play an important role in soil pH management. Lime is the most commonly used product for adjusting soil pH, but it functions like a steering wheel that turns only one way: lime can only raise pH, making soils less acidic.

While many cropping systems require lime to counteract acidification from fertilizer use and crop removal, some soils are naturally alkaline, with a pH greater than 7. In these situations, sulfur can be a useful tool.

Elemental sulfur can be used to lower soil pH across a field. As soil bacteria oxidize sulfur, sulfuric acid is formed, increasing soil acidity. However, the quantities required often make this approach cost-prohibitive on a field-wide basis.

A more commonly used sulfur source is ammonium thiosulfate (ATS), which is frequently applied with liquid nitrogen. ATS rapidly converts to sulfate, a plant-available form of sulfur, and also acidifies the soil to a modest degree. When placed near the root zone in alkaline soils, ATS can improve sulfur availability while also lowering pH in the immediate root environment, potentially enhancing nutrient availability and crop yield—even if the overall field pH remains unchanged.

Managing Sulfur for Profitability

Sulfur is an essential plant nutrient, but it can be difficult to measure accurately through soil testing because it is highly soluble and mobile in the soil profile. Unlike in the past, sulfur is no longer a “free” input from atmospheric deposition, making it increasingly important to evaluate sulfur needs intentionally.

For these reasons, Halderman Farm Managers carefully assess sulfur availability and incorporate it into fertility programs where appropriate and profitable, ensuring balanced nutrition, efficient fertilizer use, and optimal crop performance.